Transmission of nerve impulses

Hyderabad: This is in continuation to the last article focusing on neural system and mechanisms of neural coordination in a human body. In today’s article we continue to learn more about how nerve impulses are generated, conducted and transmitted.

Generation and conduction of nerve impulse

In the last article, we discussed learnt that the electrical potential difference across the plasma membrane at the site A is called the action potential, which is, in fact, termed as a nerve impulse.

• At sites immediately ahead, the axon (e.g., site B) membrane has a positive charge on the outer surface and a negative charge on its inner surface.

• As a result, a current flows on the inner surface from site A to site B.

• On the outer surface, current flows from site B to site A to complete the circuit of the current flow.

• Hence, the polarity at the site is reversed, and an action potential is generated at site B.

• Thus, the impulse (action potential) generated at site A arrives at site B.

• The sequence is repeated along the length of the axon and consequently the impulse is conducted.

• The rise in the stimulus-induced permeability to Na is extremely short-lived. It is quickly followed by a rise in permeability to K .

• Within a fraction of a second, K diffuses outside the membrane and restores the resting potential of the membrane at the site of excitation and the fibre becomes once more responsive to further stimulation.

Transmission of impulses

• A nerve impulse is transmitted from one neuron to another through junctions called synapses.

• A synapse is formed by the membranes of a pre-synaptic neuron and a post-synaptic neuron, which may or may not be separated by a gap called synaptic cleft.

• There are two types of synapses, namely, electrical synapses and chemical synapses.

• At electrical synapses, the membranes of pre- and post-synaptic neurons are in very close proximity.

• Electrical current can flow directly from one neuron into the other across these synapses.

• Transmission of an impulse across electrical synapses is very similar to impulse conduction along a single axon.

• Impulse transmission across an electrical synapse is always faster than that across a chemical synapse.
• Electrical synapses are rare in our system.
• At a chemical synapse, the membranes of the pre- and post-synaptic neurons are separated by a fluid-filled space called synaptic clef.
• Chemicals called neurotransmitters are involved in the transmission of impulses at these synapses.
• The axon terminals contain vesicles filled with these neurotransmitters.
To be continued…

By
Dr. Modala Mallesh
Subject Expert

Palem, Nakrekal, Nalgonda

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